Interpretive Summary: The time domain reflectometry (TDR) technique for measurement of soil water content has become increasingly accepted, and several systems for automatically collecting water contents from many soil probes have been proposed. To keep costs low and to automate data collection, a device called a multiplexer is needed to switch the input of the expensive TDR instrument from one probe cable to another. A 16-channel multiplexer was developed that could be controlled by the parallel port of a small computer. Up to 16 of these multiplexers may be used in one system to collect water contents from up to 240 soil probes. The water contents may be used to make irrigation control decisions, for environmental monitoring, for road bed stability assessment, etc. The data collected may also be used to assess the relative salinity of soils. Testing of five multiplexers showed that water content and salinity data were not affected by replacing one multiplexer by another or by using different channels of the multiplexers. Power consumption was low, averaging 0.054 Amp at 12 Volts. The multiplexers were successfully used over a three-year period in the field. A Cooperative Research and Development Agreement with Dynamax, Inc. of Houston, Texas, has brought the multiplexer to market.

Technical Abstract:
Automatic systems for collecting time domain reflectometry (TDR) wave forms from many soil water content probes require a multiplexer to connect these probes to a TDR instrument. A 16 channel, 50 ohm coaxial multiplexer was developed and tested. Control is by synchronous serial addressing using TTL level signal lines, as in the parallel port of a personal computer. Jumper selectable addresses allow up to 16 multiplexers (240 probes connected) to be controlled by three lines. Testing 5 multiplexers showed that quiescent power consumption was 6 mA at 12 VDC, peak power consumption was 101 mA, and mean power consumption during switching was 54 mA. Wave form placement on a cable tester screen was not affected by the multiplexer channel used. The standard deviation (SD) of horizontal placement was 0.012 ns which compared favorably to the 15.6 ns full screen width of the wave form (5 multiplexers by 16 channels each = 80 measurements; 30 cm probe length). Input channel also had negligible effect on travel times of the TDR pulse in a water content probe. The SD of 0.011 ns for a mean travel time of 8.68 ns was only slightly larger than the SD of 0.007 ns obtained when 80 wave forms were captured using the first channel of one multiplexer. Corresponding SD values for water content were 0.0006 and 0.0004 m**3/m**3, respectively. The multiplexer used, and the multiplexer channel, had no important effect on the wave form voltage levels needed for calculation of bulk electrical conductivity (BEC). The ratio of final voltage to incident voltage, which is directly proportional to BEC, had a SD of 0.0002 when measured 80 times on channel one of one multiplexer and a SD of 0.0006 when measured once on each of the 16 channels of 5 multiplexers. The multiplexer can be used to measure soil volumetric water content and BEC.